Forgeable high strength austenitic alloy with copper, molybdenum, tantalum and nitrogen additions



Feb. 25, 1958 EBERLE ETAL 2,824,796

FORGEABLE HIGH STRENGTH AUSTENITIC ALLOY WITH COPPER, MOLYBDENUM, TANTALUM AND NITROGEN ADDITIONS Filed Jul 50, 1954.

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/ 3 g g (830, a F a In "I m N INVENTORS o g 9 Erz'zz ZZZer/e 7 TS?! HLSINHELLS aamdna BY C/arZ Z. Corey ATTORNEY Unite tare 2,824,796 Patented eh. 25,1958

fine FORGEABLE HIGH STRENGTH AUSTENITIC ALLOY WITH COPPER, MDLYBDENUM, TANTALUM AND NHRGGEN ADDITIONS Application July 30, 1054, Serial No. 446,877

8 Claims. (Cl. 75-125) This invention relates to forgeable alloy steels having enhanced stress rupture strength, corrosion resistance, and freedom from embrittlement in an extended service, at elevated temperatures and stresses, and, more particularly, to a fully austenitic chrome-nickel-iron alloy steel attaining the foregoing properties with a minimum total alloy content.

For a number of years there has been a steady increase in the superheater outlet temperatures and pressures of vapor generators, with a resulting increase in the efiiciency and economy of turbines driving electric generators. These temperature and pressure increases required alloy steels to be used in the superheaters, such as stainless steels of the columbium and titanium bearing 18C1'8Ni AISI Types 347 and 321. With superheater outlet temperatures of 1050 35., the pressures involved are frequently substantially in excess of 2000 p. s. i.

With pressures of this order, the superheater tubing must have wall thicknesses of up to for such l88 alloys to remain within their allowable working stresses. Such wall thicknesses are undesirable, not only from the standpoint of fabrication problems but also from the standpoint of heat transfer and thermal stress gradients across the wall of the tubing. As a consequence, the increase in superheater outlet temperatures recently has been arrested at substantially the 1100 F. level.

Any further substantial increase in superheater outlet temperatures requires steel alloys capable of practical fabrication into tubing having wall thicknesses acceptable from the fabrication, heat transfer, and thermal stress gradient standpoints, and having long-time strength and corrosion resistance at temperatures in excess of 1350 F. and pressures substantially in excess of 2000 p. s. i. In addition, considering the large quantities of such tubing required in modern vapor generator installations, such alloys must have a low total alloy content in order to be economically feasible for use as superheater tubing.

There are known alloys which have long-time strength at high temperatures but which either have too high an alloy content to be economically practical for use as superheater tubing or are substantially non-forgeable, difiicult to forge, or characterized by a loss of desirable properties when exposed for long times to temperatures used in superheater service.

The present invention is, accordingly, directed to a steel alloy capable of economically practical use as tubing operating at temperatures in excess of 1350 F. and pressures in excess of 2000 p. s. i., and having the lowest possible alloy content, being particularly low or lean in strategically important elements. The invention is particularly directed to such an alloy meeting the following requirements:

(1) Stress-rupture strength, at 1350 F, at least twice that of A181 Type 304 alloys, the most economical steel alloys commercially available for use at such elevated temperatures;

(2) Adequate resistance to corrosion by superheated vapor and combustion gases at 1350 F;

(3) Adequate hot plasticity, for fabrication tubing;

(4) Favorable mechanical properties;

(5) Weldability; and

('6) Freedom from serious embrittlement in long-time service at such high temperatures.

To meet these requirements, the invention alloy has the following base composition:

into

. Percent Cr 15.00-20.00 Ni 12.00-18.00 C as 0.02 0.15 Mn 0.25- 2.50 Si 0.10

Balance iron with the usual impurities.

This base composition is a fully austenitic iron-chromenickel steel alloy of relatively low carbon and silicon content. The chromium content is sufiiciently high for adequate oxidation and corrosion resistance at temperatures of the order of about 1500 F, and yet sufliciently low to suppress sigma-phase formation. The nickel content is sufficient to maintain the alloy structure fully austenitic over a wide range of variation in alloying additions. The fully austenitic, or face-centered lattice, structure is important for maximum sustained hightemperature strength, the low carbon content assures hot plasticity and weldability, and the low silicon content is adequate insurance against micro-fissuring in welding.

The creep-rupture strength of this base composition is raised by suitable alloy addition designed to produce age hardening processes in the base composition by forming complex carbides or intermetallic compounds which are soluble in the base composition at very high temperatures but insoluble or of limited solubility therein at lower temperatures in the general vicinity of the contemplated use temperature; i. e. of the order of 1350 F. or higher.

In accordance with the present invention, the creep rupture strength of the base composition is very substantially increased by adding thereto Cu from 2.00% to 3.00%, molybdenum from 2.00% to 3.00%, tantalum from 0.50% to 2.00%, and nitrogen from 0.15% to 0.25%. The invention alloy may be classed generally as a l5Cr15Ni-2.5Cu-2.5MolTa-.lSN steel alloy.

In the drawing, the single figure is a graphical comparison, at 1350 F, of the creep rupture strength of the invention alloy and an AlSI Type 304 18Cr-8Ni steel alloy.

In the invention alloy, the chromium content selected had to be high enough to insure adequate resistance to oxidation and scaling at a contemplated maximum use temperature of 1350 F. to 1450" F., and low enough to inhibit or minimize the formation of embrittling sigma phase. A chromium content of 15% to 17% is suitable for efiecting these results. It is advisable to hold the chromium content on the low side since chromium, as well as most of the other elements available for strengthening the base composition, is a ferrite former promoting the weak, body-centered cubic lattice structure which has to be compensated by suitably increased additions of the relatively expensive, and strategically important, austenite forming nickel. The nitrogen addition aids in stabilizing the strong austenite structure and is also significant in the age hardening reaction.

With a chromium content of 15% to 17%, a nickel content of 15% is suiiicient' to neutralize'the ferrite forming tendencies of chromium and of the precipitate producing and strengthening additions. A chromium content of 15 to 17% with a nickel content of about 15% is advantageous from the standpoint of creep strength, as

is infen'orto'that' of nickel. V

ganese content is 1.75%, which is near the upper'end of i composition.

' investigations demonstrate thatan increase in nickel content above 8% and 10%, required to'achieve a fully Iaustenitic structure in a 20% chrome-iron alloy, does not improve the creep strength in any significant degree.

Similarly, if the; nickel content in such an alloy is held ;at an increase in the chromium content from 15% V to 251% does not add inaterially'to the creep strength. The carbon content of the invention alloy iscarefully selected to assure ,a high enough carbon content for strengthening the alloy through formation of complex carbides yet not so high as to afiect adversely forgeebility and lead to seams in tubing formed from the alloy. For

this purpose, the carbon content has a maximum of 0.12% and preferably is 'held between 0.03% and 0.05%.

Manganese has a beneficial'eifect upon hot working properties due to its action upon oxygen and sulphur. It is also desirablea's an ingredient due to its'tendency to form 'austenite, although its potency, in'this respect,

Hence, the preferred manthe range of manganese commonly found in 188 type 'Siliconis a strong ferrite former, and should be lk e'pt.

at a low value where his desired to promote austenite formation. On the'other hand, silicon participates in the formation of strengthening compounds, such as silicides,

with columbiurn :and tantalum. It is also a powerful deoxidizen andenhances resistance to oxidation, at high temperatures, by forming a tightly adherent'protective scale, being' rnuch. more effective than chromiumdnthis respect. By combining a' 'low-range chromium content Zwith ahigh range silicon content, satisfactory scaling resistance, with a minimum tendency to sigma-phase'emtent. of substantially 0.75% is preferred.

As stated, the base composition is strengthened by.

brittlement, is assured. For these'reasons, a silic'onc'onalloy'additions designed'to produce age hardening'processes. For this purpose, the invention alloy, including such additionsyis solution heat treated at a high tempera .ture, as 2200 Fto-2300 F., followed by an aging treat ment, or'by use, at a lower temperature, such as 1350 F. )With suitable alloy additions, a'fine dispersion 'of'precipitated compounds in the lattice structure of the matrix is achieved. This fine dispersion resists or retards plastic deformation .understress at elevated temperatures, and

' 'thus'produces high load carrying ability at such elevated I temperatures. 7 V a In accordance with the present invention, it found that, of available strengthening alloy additions,

tantalum and molybdenum are most potentin improving the'r'upture strength, at elevated temperatures, of the base Copper, when used with tantalum V and molybdenum, is very eitectivein obtaining optimum stresshas been 0 Mo s. zoo-3.00%.

Balance iron with the usual impurities.

The preferred Cu, Mo, Ta, and N contents are, respective- 7 1y, 2.50%,=2.50%, 1.00%, and 0.15%. V J

Alloys of substantially this preferred composition have been compounded, solution heat-treated at 220031 2300 F. andfaged at temperatures of the order 0f'1'300" F.1500 F.- These alloys'have been subjected to stress I rupture testsat 1350 E; for over 10,000 hours.- In the accompanying" drawing, the stress-rupture curve, plotted on a logarithmic scale with rupture strength in' p. 's. i. V as'ordinates and hours under stress as abscissae, is given 7 V with actual values up to the 10,000 hourspoint, and extrapolated to 100,000hours, the'test' temperature being 1350 F.

Curves A and A represent the stress-rupture values 1 of alloys embodying the-invention, while curve-B repre' sentsthose of an AISI Type 3.04 l8Cr 8Niall oy. 'Itv/ill be observed that, at 1000 hours, the stress-rupture strength of the invention alloys is 20,500'22,500 p.s, i., at-least.

2.5 times the strength of the Type 304 alloy. At 10,000

hours, the stress-rupture strength of the invention alloys is 14300-11800 p. s. i.,'as compared to about 4700 p. s, i.

for the Type304' alloy. At 100,000 hours,-the indicated 7 rupture strength ofthe invention alloys is 10,000--1'1,000

p. s. i., nearly four [(4) times the 2700 pus. i. value for the-Type 304 alloy. The valuesfor the A181 T ype 304 oy are taken from ASTM-ASME Spec. Tech; Pnbl, N0; j 7 V H positions Within the following ranges: V a

or Q 1 r 14.90-18.00

rupture strength at high temperatures. 'Nitrogen hasheen found to be notably efiective as a strengthening additive when used in combination with tantalum andmolyb: denum. K

It has been found further that the strengthening effect I -of'in'dividual additives is not. additive and'does not even cor relate with the relativeindividual potencies when the additives are added incornbination to the basecompos i- 'ti on. There appears to be an optimum concentration for eachelement Which'appears to differ for dinerent cornbinations'of additives. V

: Within jthe composition rangeipreviously tabulated, a

a "preferred composition. of a iforgeable, high-strengthathigh-temperature alloy embodying the' invention, and l 'which is lean in alloy content and economically practical for superheater tubing, is as follows; i K V 17.00% mairimum.

Ni V 15.00% maximum.

Mn 7 2.00% maximum. 0.75% maximum.

' zoo-3.00%.

' p of the invention principles, it will'be understood that the V: invention maybe embodied otherwise without departing" 7 While a specific'ernhodi'ment of the invention has been shown and described in detail to illustrate the application from such principles. :1

Q 7 What is claimed is:

11- A 'forgeable austenitic steel under stress at temperatures'of-ithe order .0f 1 300-- said alloy having the following composition:

ori- ;1500- 2000I .N" -;i2.00+ s. 0.

Percent i alloy. having superior i v stress resistance and corrosion resistance properties, and; 7 a freedom from impacternbrittlement, in ex tendedservice 1 Cr 17.00% maximum. Ni 15.00% maximum. C 0.12% maximum. Mn 200% maximum. Si 0.75% maximum. Cu 2.00-3.00% Mo 2.00-3.00% Ta 0.502.00% N 0.15-0.25%

Balance iron with the usual impurities; said alloy having a rupture strength, after 100 hours under stress at 1350 F., of at least 28,000 p. s. i. and, after 1000 hours under stress at 1350 F., of at least 20,500 p. s. i.

4. As an article of manufacture, a tube fabricated from the alloy defined in claim 3.

5. A forgeable austenitic steel alloy having superior stress resistance and corrosion resistance properties, and freedom from impact embrittlement, in extended service under stress at temperatures of the order of 1300 F.; said alloy having the following composition:

6 Ta 1.00% N 0.15%

Balance iron with the usual impurities; said alloy having a rupture strength, after hours under stress at 1350 F., of at least 28,000 p. s. i. and, after 1000 hours under stress at 1350 F., of at least 20,500 p. s. i.

6. As an article of manufacture, a tube fabricated from the alloy defined in claim 5.

7. A forgeable austenitic steel alloy having superior stress resistance and corrosion resistance properties, and freedom from impact embrittlement, in extended service under stress at temperatures of the order of 1300 F.; said alloy having the following composition:

Percent Cr 14.90-18.00 Ni 14.00-15.15 C 0.12 Mn 0.80-1.75 Si 0.40-0.88 Cu 2.97-3.00 Mo 250-300 Ta 0.50-1.39 N 0.15-0.25

Balance iron with the usual impurities; said alloy having a rupture strength, after 100 hours under stress at 1350 F., of at least 28,000 p. s. i. and, after 1000 hours under stress at 1350 F., of at least 20,500 p. s. i.

8. As an article of manufacture, a tube fabricated from the alloy defined in claim 7.

References Cited in the file of this patent UNITED STATES PATENTS 2,540,509 Clarke Feb. 6, 1951 FOREIGN PATENTS 478,014 Italy Feb. 12, 1953 670,555 Great Britain Apr. 23, 1952 908,191 France Apr. 2, 1946 

1. A FORGEABLE AUSTENITIC STEEL ALLOY HAVING SUPERIOR STRESS RESISTANCE AND CORROSION RESISTANCE PROPERTIES, AND FREEDOM FROM IMPACT EMBRITTLEMENT, IN EXTENDED SERVICE UNDER STRESS AT TEMPERATURE OF THE ORDER OF 1300* F.; SAID ALLOY HAVING THE FOLLOWING COMPOSITION: PERCENT CR -- 15.00-20.00 NI -- 12.00-18.00 C -- 0.02-0.15 MN -- 0.25-2.50 SI -- 0.10-1.00 CU -- 2.00-3.00 MO -- 2.00-3.00 TA -- 0.50-2.00 N -- 0.15-0.25 BALANCE IRON WITH THE USUAL IMPURITIES; SAID ALLOY HAVING A RUPUTURE STRENGTH, AFTER 100 HOURS UNDER STRESS AT 1350*F., OF AT LEAST 28,000 P. S. I. AND, AFTER 1000 HOURS UNDER STRESS AT 1350*F., OF AT LEAST 20,500 P. S. I. 